1. Field of the Invention
The present invention generally relates to data recording/reproducing apparatuses, and more particularly, to a data recording/reproducing apparatus having a substituting part substituting for a burst error and to a method of substituting for a burst error.
2. Description of the Related Art
Apparatuses that record and reproduce data include various recording/reproducing apparatuses, such as recording/reproducing apparatuses of magnetic disks, magnetic tapes, optical disks, magnetic optical disks, and the like. In order to record data on such media, magnetic recording marks are mainly used. It is possible to save data permanently and at lower cost than semiconductor memories by magnetic recording. Nowadays, recording/reproducing apparatuses are essential as information recording apparatuses for computers, for recording such as images and image information having a lot of information.
FIG. 1 shows the construction of a conventional data recording apparatus.
First, a description will be given of a case where data are recorded. User data Uk are input to an encoder 101 that modulates the user data Uk to codes that can be iteratively decoded. Then, data interleaved via a puncture part (MUX puncture) 102 and an interleaver (π) 103 are supplied to an LD driver 104. The LD driver 104 modulates a laser beam based on the supplied data and records the data on an information recording medium 105. In an example shown in FIG. 1, a magnetic optical disk is used as the information recording medium 105 (hereinafter referred to as the “magnetic optical disk 105”). However, a magnetic disk, an optical disk, and other information recording media may also be used. In the case of a magnetic disk, the data are supplied to a magnetic head suitable for the recording medium.
Next, a description will be given of a case where data are reproduced from the magnetic optical disk 105. Recording marks are reproduced from the magnetic optical disk 105 by a head and a MO reproduction signal is obtained. A recording/reproducing system 106 constructed by a writing head, the magnetic optical disk 105, and the reproducing head forms a partial response channel (PR channel) having characteristics such as PR (1, 1). The reproduced MO reproduction signal is amplified by an amplifier 110. Then, the amplitude of the signal is controlled by an AGC 111, and thereafter waveform equalization is performed on the signal by a low-pass filter (LPF) 112 and an equalizer (EQ) 113. The MO reproduction signal Yi subjected to waveform equalization as described above is converted to a digital signal by an A/D converter 114 by using a clock synchronized with the reproduction signal. Then, the digital signal thus converted is accumulated in a memory 115.
Next, based on the data accumulated in the memory 115, the user data are reproduced by a iterative decoder 116 such as a turbo decoder. The iterative decoder 116 is controlled by a controller 117 (for example, an ODC in the case of a magnetic optical disk apparatus). The iterative decoder 116 decodes the user data through iterative decoding of the number of times determined by the controller 117.
FIG. 2 shows an example of the encoder 101 that encodes the user data into codes for performing iterative decoding. The encoder shown in FIG. 2 is an iterative convolutional encoder and is constructed by registers 201 and 202, and exclusive ORs 203 and 204. The encoder shown in FIG. 2 generates a parity sequence pk from the user data sequence Uk. 
FIG. 3 shows an example of a conventional construction of the iterative decoder 116 in FIG. 1. Data (a reception signal sequence) yi represent a reception signal digitized by the A/D converter 114 and accumulated in the memory 115 shown in FIG. 1. The sampling data yi are supplied to an a posteriori probability decoder (PR Channel APP) 301. The a posteriori probability decoder 301 calculates, under the condition where input sampling value Y (y1, y2, y3, . . . yn) is detected, a logarithmic likelihood ratio L(ci*) between the probability P (ci=1|y) that the next input bit ci is “1 ” and the probability P (ci=0|y) that ci is 0. When iteration is made for the first time, a priori information La(ci) input to the a posteriori probability decoder 301 is all zeros. This represents that the probability that all of the bits ci are “1” and the probability that all of the bits ci are “0” are the same probability (are equal).
Then, the a priori information La(ci) is subtracted from L(ci*), which is the output of the a posteriori probability decoder 301, by a subtractor 302 so as to obtain extrinsic likelihood information Le(c). The extrinsic likelihood information Le(c) is converted by a deinterleaver 303 and thereafter sent to a depuncture part 304. The depuncture part 304 converts the deinterleaved extrinsic likelihood information Le(c) to likelihood information L(uk) corresponding to a data bit Uk and likelihood information L(Pk) corresponding to a parity bit Pk and supplies the information to a code decoder (Code APP) 305. The code decoder 305 outputs a logarithmic likelihood ratio L(u*) with respect to the data bit uk and a logarithmic likelihood ratio L(p*) with respect to the parity bit pk from L(uk) and L(pk), respectively. When performing iterative decoding, L(u*) and L(p*) are sent to a puncture part 306 and converted to likelihood information L(c*)(the result of combining and thinning out L(u*) and L(p*)). A priori information Le(c) is subtracted from L(c*) by a subtractor 307. Then, interleaving is performed by an interleaver 308 on the output of the subtractor 307 so as to obtain La(ci). La(ci) is supplied to the a posteriori probability decoder (PR Channel APP) 301 as a priori information and iteration is repeatedly performed. Data detection is performed such that a hard decision part 309 determines whether L(u*) obtained from the code decoder 305 is “1” or “0” and outputs the user data sequence Uk. 
However, the above-described conventional example suffers from the following problems.
Generally, there are local defects in recording media such as optical disks (including magnetic optical disks), magnetic disks, and magnetic tapes. Especially, in optical disks and magnetic tapes that are replaceable media, defective parts are increased by the influence of adhesion of dust and scratches made when handling them. The iterative decoding described above operates very effectively for reduced SNR associated with recording media and apparatuses of higher density. When a reproduction signal (burst error signal) of a defective part in a recording medium is input, however, likelihood information that is made vastly different via a priori information is propagated to data of a part(s) other than the burst error part, and an error in the burst error part is propagated to the data of the other part(s). This is because the likelihood information obtained from the data of the burst error part is greatly different from the likelihood information obtained from the original data. Hence, there is a problem in that the effect of error correction by iterative decoding cannot be obtained sufficiently.